The present invention relates to a linear actuator for a random access memory apparatus or the like and, more particularly, to such a linear actuator which provides highly accurate and fast positioning of a data transfer head with respect to a disc recording surface.
Direct access memories of the type employing one or more magnetic discs for recording and storing data are widely used as peripheral memories for computers and other data processing units. Memories of this nature have the advantage of enabling information to be either transferred to, or removed from, randomly selected address locations or tracks on the disc without the necessity of the memory having to serially seek the desired location as is necessary, for example, when magnetic tape memories are employed.
The read/write recording head or other data transfer device associated with each disc recording surface for transferring data to and from such surface is typically movable translationally with respect to the disc between the various address locations. It will be recognized the speed at which data can be applied or recognized that from a disc will depend in a large measure on the speed with which the transfer head is movable between various ones of the locations. It is also necessary that the transfer head be quite accurately positioned with respect to each address location, not only to assure that data is either applied to or removed from the proper address, but to enable address locations to be closely spaced to one another to provide adequate storage capacity on the disc surface.
Typically, the transfer head or heads are mounted on a carriage which is, in turn, mounted upon a track for travel on a translational path which will provide the desired movement of the heads between the radially spaced address locations. The carriage can be driven by various mechanisms, the most satisfactory of which to date is basically a voice coil, i.e., a solenoid of the type used to drive an audio speaker. While linear actuators of this type have been generally satisfactory, they do have certain disadvantages. For one, it will be appreciated that every time it is desired to move a transfer head and, hence, the carriage supporting the same, it is necessary that the momentum of the carriage be changed from essentially zero to a high value in a very short time in order to achieve a realistic access time. The electrical power required to cause a voice coil to provide the accelerations required is of a high order of magnitude. Moreover, a voice coil actuator is relatively inefficient in terms of the amount of electrical power that must be delivered to it relative to the mechanical power output. Also, complicated drive and control circuitry has been required to provide the necessary power and yet enable quite accurate positioning of the head or heads at address locations.